It is already known that a plane array of acoustic transducers organized in rows and in columns and oriented in a common direction perpendicular to the plane of the array can be used for emitting acoustic signals in various different directions and for receiving signals that are backscattered by the fluid. The principles on which the control and operation of an array of that type are based have already been described, in particular in U.S. Pat. No. 3,448,450 for a radar application, in French patent 2,499,252 in a sonar application, and in U.S. Pat. No. 4,558,594 in a "sodar" or acoustic "radar" application for determining wind velocity in the atmosphere.
In devices of that type, it is conventional to emit signals at known constant frequency in a determined direction, and then to pick up the signals that are backscattered in that direction, for the purpose of determining their frequency and of deducing the velocity of the fluid in the above-specified direction on the basis of the difference between the frequency of the emitted signals and the frequency of the signals as picked up (Doppler effect). Thus, by emitting and picking up acoustic signals successively in three different directions, it is possible to determine the three-dimensional velocities of wind in the atmosphere or of water in a current in the sea, for example.